The customary electrode structure used in high pressure sodium vapor lamps consists of a tungsten shank with tungsten coils wound thereon which are generally sealed at each end of a tubular light-transmitting ceramic envelope that further contains an ionizable filling. Lamps of this general type are disclosed in U.S. Pat. Nos. 4,025,812 and 4,065,691, both assigned to the present assignee.
The ceramic arc tube is supported within an outer vitreous envelope or jacket of elongated shape generally provided at one end with the conventional screw base. High pressure sodium vapor lamps are also generally vacuum jacketed in order to conserve heat and maximize efficiency.
The high cost and difficulties experienced with manufacture of the customary all-tungsten electrode structures for high pressure sodium vapor lamps advises suitable replacement of this refractory metal to the extent permitted by the severe lamp operating conditions. The tungsten wire customarily used for manufacture of the coil elements in said electrode structure is prone to surface and internal defects which can produce wire breakage and machine wear during electrode fabrication as well as further difficulties of various kinds in the lamp operation. On the other hand, tungsten is recognized to withstand the very high electrode operating temperatures and corrosive environment that is experienced within the arc tube of these lamps. While some sputtering is experienced with an all-tungsten electrode structure during lamp operation, these electrodes provide an acceptable lumen maintenance over considerable time periods.